PROJECT SUMMARY/ABSTRACT Breast density is a risk factor for developing breast cancer and decreases the accuracy of screening mammography. An estimated 27 million women aged 40-74 in the U.S. have dense breasts and they experience elevated rates of advanced stage breast cancer diagnoses associated with poor outcomes. Thirty- seven states now require mammography facilities to notify women with dense breasts of the limitations of mammography and recommend discussion of screening options with their healthcare providers, and a national law is pending. In the absence of screening guidelines for women with dense breasts, there has been a dramatic increase in use of supplemental ultrasound screening, which is widely available and has low direct medical costs. Early studies of supplemental ultrasound performance suggest increased cancer detection but high rates of false positive exams leading to unnecessary biopsies. The United States Preventive Services Task Force has called for studies that evaluate the impact of supplemental ultrasound screening on meaningful clinical outcomes, such as advanced cancer rates, to inform screening guidelines for women with dense breasts. We recently demonstrated that mammography screening failure rates (i.e., advanced cancers and interval cancers after a normal mammogram) among women with dense breasts vary widely according to clinical risk factors. Therefore, we propose to assess supplemental ultrasound screening performance within a new risk-based framework. We hypothesize that supplemental ultrasound screening targeted to the subset of women with dense breasts at high risk of mammography screening failures will yield a favorable benefit-to- harm profile. We will use observational data from more than 100,000 screening ultrasound exams and 2 million mammography screening exams collected via the Breast Cancer Surveillance Consortium to (Aim 1) examine the test performance of supplemental screening ultrasound according to technique (handheld vs. automated) and type of primary screening (digital mammography vs. digital breast tomosynthesis); and (Aim 2) evaluate supplemental screening ultrasound outcomes across levels of risk for mammography screening failures. These results will be used as inputs in two simulation models from the Cancer Intervention and Surveillance Modeling Network to (Aim 3) evaluate the long-term benefits, harms, and costs of supplemental ultrasound strategies targeted to women at high risk of mammography screening failures. Our study will be the largest evaluation of supplemental ultrasound and the first to evaluate rates of interval and advanced cancers according to risk of mammography screening failures. Our results will provide urgently needed, actionable evidence for women, healthcare providers, and guideline-makers evaluating screening options for women with dense breasts. This evidence will support effective supplemental screening strategies that reduce the burden of breast cancer among women for whom mammography screening is not adequate, while minimizing potential harms.